The present invention relates to electrophotographic processes. In particular, it relates to a process for making fuser belts used to fix toner in such processes.
In electrophotography, a latent image is created on the surface of an insulating, photoconducting material by selectively exposing an area of the material""s surface to light. A difference in electrostatic density is created between the areas on the surface exposed and those unexposed to the light. The latent electrostatic image is developed into a visible image by electrostatic toners which contain pigment components and thermoplastic components. The toners, which may be liquids or powders, are selectively attracted to the surface of the photoconductor, either exposed or unexposed to light, depending upon the relative electrostatic charges on the photoconductor surface, the development electrode and the toner. The photoconductor may be either positively or negatively charged, and the toner system similarly may contain negatively or positively charged particles.
A sheet of paper or intermediate transfer medium is given an electrostatic charge opposite that of the toner and then passed close to the photoconductor surface, pulling the toner from that surface onto the paper or intermediate medium still in the pattern of the image developed from the photoconductor surface. A set of fuser rolls or belts, under heat, melts and fixes the toner in the paper subsequent to transfer, producing the printed image.
The electrostatic printing process, therefore, comprises an intricate and on-going series of steps in which the surface of the photoconductor is charged and discharged as the printing takes place. In addition, during the process, various charges are formed on the photoconductor surface, the toner and the paper surface to enable the printing process to take place. Having the appropriate charges in the appropriate places at the appropriate times is critical to making the process work.
After the image is transferred to the paper or other recording medium, it goes to the fuser where the paper is moved through a nip where it is heated and pressed. This melts the thermoplastic portion of the toner, causing it to bond with the fibers of the paper, thereby fixing the image onto the paper or recording medium. In the past, the majority of fuser assemblies used a fuser roll. These fuser rolls are typically aluminum cylinders with a heating lamp inside and a release coating on the outside. In this system, paper or transparency film with a toner image on it is passed through the fusing nip formed between the fuser roll and a backing roll. As the toner image passes through the fuser nip, the heat and pressure fuse the toner image to the paper or transparency film. This system has been used for many years because of its simplicity and functionality in high speed systems. The problem with this system, however, is that it requires the fuser roll to preheat before it may operate. This is true even with the machine in standby mode between printing or copying jobs. The need for preheating is the result of the large heat capacity of fuser roll. This preheating means a delay for the user before each printing or copying job can be started.
In recent years, a new on-demand fusing system has been developed and is being used in the industry to minimize this delay problem. This system is mainly composed of a ceramic heater and a thin film belt through which the heat is transferred to the toner-laden image. The ceramic heater is thin and has a small heat capacity compared to the fuser roll. The fuser belt is designed to conduct the heat from the heater to the toner image with minimal resistance. The contact of the belt with the paper going through the fuser nip is the driving force for the belt to turn around the ceramic heater. The nip for the fuser belt system is formed by the fuser belt and a backing roll. The net result is a system which essentially eliminates any significant time delay caused by initial heating of the fuser system.
The fuser belt described above is typically composed of three layers. The first layer is a polymer film. This polymer film is the main substrate which gives the fuser belt structural integrity. The film must have specific properties regarding flexibility, as well as physical tolerances to high temperatures and repeated heating and cooling cycles. The polymer film must also be a good conductor of heat. The polymer chosen for this film is typically a polyimide with a material, such as boron nitride, dispersed within it for improved heat conduction. The current industry standard for producing this polymer film is a vertical dip coating technique carried out on a specially coated metal mandrel. After the polymer has been cured, it is slipped off the coating mandrel and is coated with one, two or more layers to form the finished belt.
The second layer of the fuser belt typically is a conductive primer coating. This layer provides a path within the finished belt for static charge dissipation. At one end of the fuser belt, there is a strip of exposed conductive primer, providing a place to ground the belt and to remove the static charge generated during belt operation. This is important because there can exist an electrostatic offset phenomenon whereby the toner is electrostatically transferred from the paper or transparency film to the fuser belt. This is undesirable since, when it happens, the toner transferred previously as a result of the offset is then fused to the print copy. This phenomenon produces a ghost-like image that deteriorates the overall image quality. The grounding clip, which is in contact with the strip of the exposed conductive primer coating, provides a path for charge dissipation and thus reduces the electrostatic offset phenomenon.
The third layer of the fixing belt is composed of a release coating. A primary purpose of this coating is to provide a surface to which the toner will not adhere during fusing. A second purpose of this coating layer is to provide a strong wear layer for the belt. As stated above, the preferred method for manufacturing fuser belts utilizes a vertical dip coating method. A number of problems result when attempts are made to apply a typical topcoat material, such as a fluoropolymer, to the fuser belt using this vertical dip coating technique. First, the coating solution does not wet the polymer tube in a uniform sheet. Instead, the coating runs and drips off the belt during coating. This results in a topcoat which is not uniform or smooth. Second, the topcoat tends to crack when it is exposed to high temperatures (for example, greater than 350xc2x0 C.), such as during the sintering operation which is necessary for drying and curing the polymer and other coatings. Finally, the topcoat coating solution is frequently not stable for long enough to remain uniform throughout the coating process. When the solution sits for a few minutes, it frequently tends to separate into a non-homogenous solution. It therefore would be useful to develop a process which allows topcoat materials, such as fluoropolymers, to be applied to fuser belts, using the vertical dip coat technique in an efficient and effective manner. The present invention addresses that objective.
U.S. Pat. No. 5,853,892, Chen, et al., issued Dec. 29, 1998, describes the use of an amorphous fluoropolymer in the outer layer of a belt used for fusing a thermoplastic resin toner image to a substrate. The fuser belt coatings are said to require a lower sintering temperature than the conventional semi-crystalline fluoropolymers.
U.S. Pat. No. 5,778,295, Chen, et al., issued Jul. 7, 1998, describes the preparation and use of a fuser belt comprising a seamless polyimide substrate belt, a cross-linked silicone resin intermediate layer and a surface layer containing a silsesquioxane polymer.
U.S. Pat. No. 5,709,973, Chen, et al., issued Jan. 20, 1998, discloses a metal fuser belt comprising an unmatted powder-coated polytetrafluoroethylene-co-perfluoropropyl vinyl ether copolymer (PFA), an unmatted powder-coated tetrafluoroethylene-hexafluoropropylene co-polymer and an aqueous spray-coated blend of polytetrafluoroethylene and PTFE-perfluorinated vinyl ether.
U.S. Pat. No. 5,708,948, Chen, et al., issued Jan. 13, 1998, describes the preparation of a fuser belt by curing a composition comprising siloxanes whose average molecular weight ranges from 5,000 to 50,000 g/mole, the ratio of di-functional to tri-functional units varies from 1:1 to 1:2.7, and the ratio of alkyl to aryl groups varies from 1:0.1 to 1:1.2.
U.S. Pat. No. 5,547,759, Chen, et al., issued Aug. 20, 1996, discloses a fuser member comprising a metal element, a fluoroelastomer layer, a primer layer and a fluoropolymer resin layer. The fluoroelastomer layer contains a vinylidene fluoride-hexafluoropropylene co-polymer or a vinylidene fluoride-hexafluoropropylene-tetrafluoropropylene terpolymer. The primer layer contains a fluoropolymer resin and a polyamide-imide. The fluoropolymer resin layer contains a polytetrafluoroethylene-polyperfluoroalkoxy-tetrafluoroethylene and/or a polyfluoronated ethylene-propylene.
U.S. Pat. No. 5,697,037, Yano, et al., issued Dec. 9, 1997, describes a fixing member comprising a heat resistant film (preferably a polyimide film) coated with a conductive primer layer and a surface layer comprising a fluorine-containing resin and an ion-conductive material, whose melting point is greater than the maximum temperature achieved by the fixing device. The surface layer of the heat-resistant film is a polyimide, polyamide or polyphenyleneoxide. The fluorine-containing resin includes polytetrafluoroethylene, tetrafluoroethylene-perfluoroalkylvinyl ether copolymer and tetrafluoroethylene-hexafluoropropylene copolymer.
U.S. Pat. No. 5,759,655, Kitajima, et al., issued Jun. 2, 1998, discloses the preparation of a fuser belt, by coating a conducting primer layer onto a seamless layer that contains polyimide. A baked fluororesin containing carbon black is coated over the conductive primer layer. The conductive primer layer contains at least one resin selected from polyphenylene sulfide, polyethersulfone, polysulfone, polyamide, polyimide, and their derivatives. The fluororesin is selected from polytetrafluoroethylene (PTFE) polyfluoroethylene-perfluoroalkylvinyl ether copolymer (PFA), and tetrafluoroethylene-hexafluoropropylene copolymer (FEP).
U.S. Pat. No. 4,341,455, Fedder, issued Jul. 27, 1982, describes a dielectric transfer belt (not a fuser belt) which is used to transfer toner to paper in an electrostatic printing process.
U.S. Pat. No. 4,789,565, Kon et al., issued Dec. 6, 1988, teaches a method for coating a roller with a PTFE dispersion using a horizontal dip process. The procedure described does not suggest the use of isopropyl alcohol in the topcoat solution.
U.S. Pat. No. 5,309,210, Yamamoto et al., issued May 3, 1994, describes a belt fuser apparatus for use in electrophotographic processes. The belt includes a fluororesin in the belt structure. The patent contains no details of the release layer or how the release layer is put onto the fuser belt.
U.S. Pat. No. 5,397,629, Jahn, issued Mar. 14, 1995, describes a method for applying fluoropolymers onto textiles using isocyanate adhesion promoters. The patent teaches the use of thickeners, such as pectin and polyvinyl alcohol, in the fluororesin formulations. This disclosure does not deal with the manufacture of electrophotographic fuser rolls and the application of fluoropolymers to a textile surface is based on a completely different dynamic than is the dip coating of a polyimide fuser belt.
U.S. Pat. No. 5,709,949, Chen, et al., issued Jan. 20, 1998, describes spraying and dip processes for coating fluoropolymer release layers onto electrophotographic fuser members. There is no suggestion in this patent of the use of isopropyl alcohol solvent in those processes.
U.S. Pat. No. 5,765,085, Law et al., issued Jun. 9, 1998, describes an electrophotographic fixing belt which may incorporate fluorocarbons into its outer release layer. The patent includes a general disclosure of solvents which can be used in this coating process; isopropyl alcohol is not included in that list.
U.S. Pat. No. 5,789,083, Thomas, issued Aug. 4, 1998, describes a fluorocarbon primer that can be used on smooth (metal) substrates. The described process utilizes an acrylate copolymer (Primal RM-5) as a thickener. The described process does not deal with the coating of electrophotographic fixing belts or the use of acrylate homopolymers in the disclosed process.
U.S. Pat. No. 5,918,099, Schlueter, Jr., et al., issued Jun. 29, 1999, describes an electrophotographic fuser belt which incorporates a polyphenylene sulfide layer and a fluororesin release layer. There is no discussion of solvents used to facilitate the fluororesin coating process.
U.S. Pat. No. 5,922,440, Schlueter, Jr., et al., issued Jul. 13, 1999, describes electrophotographic fuser belts and includes a general disclosure on coating techniques used in the preparation of such belts.
U.S. Pat. No. 5,945,223, Kuntz et al., issued Aug. 31, 1999 describes a flow coating method for forming fixing rollers used in electrophotographic processes. In this procedure, a fluororesin is applied from a solution (not an aqueous dispersion). Numerous solvents are disclosed for use with the fluororesin; isopropyl alcohol is not among them.
U.S. Pat. No. 5,948,491, Chen, et al., issued Sep. 7, 1999, includes a general discussion of Teflon release layers which can be included in toner fuser members used in electrophotographic processes.
The present invention relates to an improvement in a method for coating a polyimide film, such as a fuser belt, with a fluorocarbon material which comprises dipping the polyimide film into an aqueous-based solution which comprises from about 30% to about 50% of said fluorocarbon and from about 5% to about 15% (preferably from about 5% to about 10%) by weight of a C1-C4 alkanol, preferably isopropyl alcohol, and drying said fluorocarbon layer.